The present invention relates to projectiles and particularly to an apparatus and method to protect extensible fins mounted in the rear portion of a projectile as the projectile is fired from a gun.
An aim of projectile design is to deliver a payload farther from the gun that fires it. This goal of longer range can be met by reducing the weight of the projectile and by increasing the size of the charge used to propel it. These two factors are not independent of one another, and increasing the propellant charge may damage the projectile because of increased gas pressure in the firing chamber and barrel. Reducing the weight may reduce the strength of the projectile to sustain the increased charge and may also reduce the apogee of the ballistic flight path, thereby reducing the range.
Projectiles have had control surfaces such as fins that fold inward to fit inside the projectile when it is fired from a gun and then fold outward once the projectile has cleared the gun barrel. Smaller fins create less drag and so allow a longer range. However, the fins must provide enough surface area to control the projectile in its flight. The required fin size can be reduced by minimizing the aftward mass of the projectile, shifting its center of gravity forward.
Past attempts at reducing the weight at the tail or aft end of a projectile by removing material from around the folded-in fins have resulted in a desirable weight reduction, but the exposed fins have been unable to withstand the concussion of being fired from a gun, especially as the charge used has been increased to increase range.
In the past, projectile fins have been mounted by a pivot pin to bosses that extend outward from the base of the projectile. Because the pressure retaining obturator has been mounted in front of the fins, the fins in their folded-in positions have been exposed to destructive pressure forces from the charge that fires the projectile.
Accordingly, there is a need for a projectile with fins pivotably mounted and able to survive the shock of being fired from a gun and that is also free of unnecessary mass at its aft end.
The present invention provides a projectile (or round) that has reduced weight in its aft end and has retractably mounted fins that are packaged to survive the shock of being shot from a gun.
A projectile utilizing the present invention has a base that forms the rearmost portion of the round. The base supports the fins and in some projectiles may form a nozzle for directing rocket exhaust to propel the projectile. An obturator is located at the front end of the base, in front of the fins.
We have discovered that the pressures inside a gun barrel behind the obturator when the charge is fired are not isostatic, but rather dynamic and turbulent. Resonances may occur in open volumes, and such resonances may cause destructive pressure waves to course through these spaces as the charge is ignited and the projectile accelerates through the barrel. The present invention limits the development of such destructive pressures while allowing a light-weight base. This is accomplished first by reducing the weight of the base as much as possible and then filling any voids in the base with an incompressible material that is lighter in weight than the metal it replaces.
The present invention may be carried into practice using a light-weight, non-metallic, substantially incompressible filler material to surround and support the fins while the projectile is in the barrel. Once clear of the barrel, the filler material may fall away, allowing the fins to extend so as to guide the projectile.
The invention may be carried out with or without using a separable, frangible sleeve or boot that surrounds the aft portion of the projectile. If such a boot is used, the volume it encloses, including the cavities housing the in-folded fins and other cavities within the boot, is filled with a filler of the kind described. The filler material may be any of a variety of materials that meet the performance specifications including high temperature grease, GE""s RTV, a wax material or any other substantially incompressible material. For ease of installation, the filler material should be flowable and, if intended, it should break away cleanly and completely from the base of the projectile. The material may also be similar in all respects to the above, but remain completely in place. With this sort of material the base may be designed as a composite structure, with the filler material bonded to the metal of the base to provide a light weight yet structurally strong base. In addition, combinations of the two types of fillers (fall away and permanently adhered) may be used.
As the projectile emerges from the barrel, the boot, if used, falls away. If the filler is designed to fall away, it too falls away. If the filler is designed to remain, then it does so. The result is that the fins are protected from concussive resonances during the first moments of acceleration as the charge speeds the projectile down the barrel.
The fins are mounted to the base of the projectile. The use of a nonmetallic, incompressible filler material allows the base to be designed to be as light-weight as possible consistent with providing the necessary strength. This results in a base with fin mounting bosses that also has many hollowed out cavities where metal not necessary for strength purposes has been removed. These cavities and the spaces around the outside of the projectile and within the envelope of its outside cylindrical shape are filled with one or more of the filler materials described above. Because the filler eliminates any voids or cavities where resonances could occur, the base of the projectile is not subjected to destructive pressure waves.